1. Field of the Invention
The invention relates to a method for regulating random accesses in a radio-assisted local network.
2. Description of the Related Art
As a rule, local networks (LANs, Local Area Networks) mean communication networks that can be extended by some 10 meters up to 10 kilometers. Radio-assisted local networks are designated as WLANs (Wireless LANs). However, the plurality of LANs only stretches over a few hundred meters inside buildings or an organization (in-house networks).
WLANs allow communication between a wired network infrastructure and mobile computers or other stations on the subscriber-side. Frequently WLANs are used as a supplement for cable-supported LANs in specific work environments. As a rule, WLANs include access points (Access Points, APs) which are connected with one another via a wire-assisted LAN and in the case of which the data transport takes place from a mobile transmitter via a radio link to the AP and is then forwarded via the LAN. The cells covered by the WLANs are with a diameter of up to a few hundred meters compared with the customary mobile radio cells so small that they are called microcells.
Examples of different standards for WLANs are HiperLAN, DECT, IEEE 802.11, Bluetooth and WATM. However, at present it seems that, as local radio-assisted networks particularly in the USA and Europe, almost exclusively those products based on the IEEE 802.11 family, have gained acceptance, in which case corresponding Ethernet connections have already been provided in many computers and portable computers (e.g. laptop, notebook, PDA) as standard practice. The radio interface defined in accordance with the IEEE 802.11b standard for accessing local networks corresponds functionally to a connection wired to local networks (LANs), which these days have developed into a standard in offices. As a rule, interface cards for radio-assisted accesses to local networks, which are also designated as NICs (Network Interface Cards), are made, from an architectural point of view, in the same way as standardized Ethernet cards and in view of our present operating systems be used per so-called plug & play. Portable computers can be retrofitted with corresponding interface cards without any problems, provided they have not already been delivered ex works with an integrated connection for a wired or a radio-assisted access to local networks. In the case of the next operating system generations (e.g. Windows XP from Microsoft), a fully integrated support of local radio networks (WLANs) is provided.
In general, the unlicensed frequency range around 2.4 GHz is used for radio-assisted WLANs. Data transmission rates are up to 11 Mbit/s. Future WLANs can be operated in the 5 GHz range and reach data rates exceeding 50 Mbit/s. With data rates of 11 Mbit/s at present and 50 Mbit/s in future, data rates are as a result made available to users of the WLANs, which are considerably higher than those data rates, which can be offered by the next, third mobile radio generation (e.g. UMTS). Therefore, for the transmission of large amounts of data, particularly in connection with Internet accesses, preference has to be given to the access to radio-assisted local networks (WLANs) for high-bit rate connections.
The standard for radio-assisted local networks IEEE 802.11 proposed in 1999 supports both the allocation of radio resources to radio stations for sending information with competition and an allocation without competition. The allocation of radio resources without competition is also used in cellular WCDMA systems. For this purpose, document US 2002/0089957 A1 describes a method in which the base station informs the mobile station at which times they may send a code to the random access channel in order to be able to compete for the allocation of radio resources by means of the base station. The allocation of radio resources without competition means that an access point allocates radio resources to radio stations, while in the case of an allocation with competition the radio stations operate a random access to the radio resources. However, within the framework of this standard, the different enquiries about resources for sending information, irrespective of the type of application being dealt with in this case are discussed here. In this way, the IEEE 802.11 standard is not in the position to guarantee a quality of service (QoS, Quality of Service). In order to remedy this shortcoming amongst others, a further development of the layer of the medium access control (MAC layer, Medium Access Control) is proposed within the framework of the future IEEE 802.11e standard. Document WO 03/026221 A1 explains the medium access methods according to IEEE 802.11. A few main features of the future IEEE 802.11e standard are described below:                The time interval between the beacon signals of the access points is fixed. A radio station is in this way no longer in the position to force the access point to a delayed sending of the beacon signal.        Data of a traffic category (Traffic Category, TC) mean the data of the MAC layer with a specific priority.        An access point can allocate radio resources to a radio station. This process is known as “polling”. In addition, a radio station can send information, in which the EDCF (Enhanced Distributed Coordination Function) is used, per random access. This is based on the CSMA (Carrier Sense Multiple Access) method, in which case an attempt is made to avoid the collisions of radio signals by using a backoff mechanism. In this case, a radio station determines, by measurements, whether or not the medium, i.e. the radio resource, is engaged. If an engagement of the medium has to be approved, a back-off time is calculated according to which a random access to the radio resources is started. Parameters of the traffic category are also included in this calculation.        The access point controls the maximum time interval (Transmission Opportunity) to be used for a random access in the EDCF mode. Each station in the radio coverage area of the access point can take this value from the beacon signal sent periodically from the access point.        In the time phases without competition (competition-free), the access point can be allocated to the radio stations by polling time intervals of different lengths for sending information.        The radio stations, which prioritize the information to be sent by you according to the TC, are called Quality of Service Stations (Quality of Service Stations, QSTA). In addition to these, there can also be radio stations in the radio-assisted local network, which do not carry out such a prioritizing.        An access point, which carries out the allocation of radio resources to the Quality of Service Stations and to other stations, as well as the regulation with regard to the allocation of radio resources in the competition, is called the Hybrid Coordinator, HC. Each Quality of Service Station has the capacities needed to carry out these tasks.        The term transmission opportunity, TXOP, on the one hand, designates the maximum time interval to be used for a random access during the competition phase in the EDCF mode. This means that should the medium be available in accordance with the regulations of the EDCF for a radio station, the radio station can use the TXOP for sending information. On the other hand, the TXOP also designates that time interval, which is made available to a radio station for sending information after radio resources have been allocated to this radio station from an access point. With a view to the competition phase, the length of the TXOP is transmitted in the beacon signal of the access point and with a view to the phase without competition, the signal, which indicates the allocating of radio resources to a radio station, contains information about the length of the TXOP in question.        
The radio coverage area of a WLAN is typically between 50 and 200 meters. If a radio station remains farther away from an access point, i.e. located outside the radio coverage area of the access point, the radio station is a remote radio station, with which no direct radio contact to the access point is possible. In order to extend the radio coverage area of a WLAN, it is possible because of this to bridge the radio signal between remote station and an access point by stations, which are at the same time located in the radio coverage areas of the access point and the remote station. Such multi-hop concepts increase the economic efficiency of WLANs, because in this case, the expensive infrastructure of a WLAN can be used more efficiently.
However, including remote radio stations in the WLANs indeed also brings along a series of problems. There is for instance the danger that remote stations on the basis of random accesses thereof, generate undesired interfering signals to the signals of other radio stations, such as for example to the important beacon signals. Such effects have to be avoided. On the other hand, a random access in view of a good utilization of the scarce radio resources should be made possible for the remote stations as often as possible. A suitable switching between rejecting and granting a random access has proved to be particularly advantageous.